In many electronic applications, there exists a need for high density electrical connections. One such means to produce high density interconnects is described in U.S. Pat. No. 4,125,310. In this patent, the two circuits to be connected have contact pads which mirror each other so the contact pads from one circuit can be brought into contact with their corresponding contact pads of the mating circuit. The contact pads of one of the circuits are raised above the plane of the rest of the circuit. When the circuits are brought together, the raised contact pads serve to concentrate the clamping force. In this way, the contact pads can be brought into contact with each other with sufficient force to ensure electrical continuity through the connection.
A variety of later means have been proposed to produce such raised bump connections. Most of these inventions involve means to make the raised bumps on a flexible circuit, where the electrical circuit is laminated to an insulating sheet such as Kapton polyimide. One such means to produce the raised bump contacts, disclosed in U.S. Pat. No. 4,116,517, involves forming the electrical circuit on the insulating sheet and then permanently deforming the circuit by some suitable tool to produce the raised bumps. Another means, described in, for example, U.S. Pat. Nos. 5,354,205 and 5,307,561 and 5,364,277, involves electroforming the circuitry with the raised bumps. In each of these patents, depressions are formed in a mandrel which is usually made of a stainless steel. These depressions can be made by any suitable tool. The locations of these depressions correspond to the intended locations of the raised bumps on the circuit. The surface of the mandrel is then selectively masked by a non-conductive coating such as Teflon. The unmasked areas correspond to the conductive traces and contact pads of the flex circuit. The masked area corresponds to the non conductive spaces around the intended circuit traces. The mandrel is then subjected to an electroforming process to electrodeposit a layer of conductive material over the entire unmasked portion of the mandrel. The electroformed circuitry, which is still attached to the mandrel, is then laminated to the insulating sheet material. The insulating sheet with the laminated circuitry is removed by a peeling operation from the mandrel. As the electroformed circuitry replicates the surface topology of the mandrel, the circuitry is produced with raised bumps corresponding to the depressions of the mandrel. After the circuitry has been removed from the mandrel, the mandrel may be reused to make additional raised bump circuits.
While the methods for producing raised bump contacts have been quite effective, at high contact and circuit trace densities these processes are prone to failure. It has been believed that these failures may be caused by stresses required to deform the pads, such as in the '517 patent. With the more recent prior art processes, involving electroforming, it is believed that the high failures rates may be induced by the stresses required to peel the laminated circuitry from the reusable mandrel and from degradation of the costly reusable mandrel. At lower contact and circuit trace densities, the wider traces can withstand these stresses without failure.
It is seen, then, that a means is needed to form high density interconnects without introducing failure-inducing stresses.